(1) Field of the Invention
The present invention relates to Flat Panel Field Emission Displays (FPFEDs), and more particularly, to a method for manufacturing a FPFED having auto gettering for eliminating outgassed material from the active electronic device area of the flat panel display.
(2) Description of the Prior Art
There is a strong need in the electronics industry for thin, lightweight display panels. For example, one application for low power, low cost flat panel displays (FPD) is in the computer industry for portable computers, such as laptop computers. The most commonly used display panel, at the current time, is the liquid crystal display (LCD), but because of the slow optical response time of the liquid crystal pixel to turn on or off (the discrete dots on the screen making up the image), and because of the relatively poor luminosity other display technologies are actively being explored.
One alternative display technology having the potential to provide the required faster response times and increased brightness is the Flat Panel Field Emission Display (FPFED), also referred to simply as a Field Emission Displays (FED). The flat panel FED can be viewed as an array of micro-miniature cold cathode electron emitters mounted on a substrate or backing plate from which emitted electrons are accelerated across the thickness of the evacuated panel to excite an cathodoluminescent material (phosphors) comprising the pixel (dot) on a transparent plate that serves as both the anode and the viewing screen. The array of very small conical shaped electron emitters are electrically accessed, by peripheral control and image forming circuits, using two arrays of conducting lines that form columns and rows. The array of column lines form the cathode contacts on which the conical electron emitters are formed. The array of row conducting lines form gate electrodes that are separated by a dielectric layer from the column lines. The column lines are formed on the backing plate, and both the row conducting lines and the insulator have opening over the column lines on which the electron emitter is formed. The edges of the openings in the row lines are in close proximity to the emitter tip, and function as the electrically addressable gate electrode or control grid for the individual electron emitters. A good review article entitled "Beyond AMLCD: Field emission displays?", by K. Derbyshire, on flat panel FEDs can be found in Solid State Technology Vol 37, No. 11, November 1994, pages 55 to 65.
The proper functioning of the field emission displays (FED) rely on maintaining an adequate vacuum within the cavity between the backing plate containing the array of electron field emitters and the transparent viewing plate coated with phosphors and serving also as the anode plate of the FED field emitters.
To better understand the problem, reference is made to the schematic cross sectional view of a prior art field emission panel (FED), as shown in FIG. 1. The cathode plate 50, containing the field emitters (not shown in FIG. 1), is separated from the anode plate 10 by sealing walls 60. Spacers 16 are usually placed between cathode and anode plates to prevent the atmospheric pressure (about 14.7 pounds/square inch) from distorting or breaking the relatively thin anode plate when the field emission panel (FED) is evacuated. The cavity 7 between the plates is then evacuated through the exhaust tube 22 by vacuum pumping means and then sealed off to maintain a high vacuum in the FED. Unfortunately, virtual leaks or outgassing from the walls and materials from which the FED is fabricated can degrade the vacuum after sealing, and thereby destroy the intended function of the FED. To achieve and maintain a good vacuum it is common practice in the vacuum tube industry to utilize a gettering material, for example, such as barium (Ba) tantalum (Ta), titanium (Ti) and zirconium (Zr) to name a few. The getter also serves as a keeper to maintain a good vacuum during the intended life of the electronic vacuum device. The gettering material 24 for the FED of the prior art, as shown in FIG. 1 is usually positioned in the exhaust tube 22. This provides a convenient means for heating, and thereby activating the localized gettering source after the exhaust tube of the FED is sealed off.
Gettering material, localized in the exhaust tube, however, is not very effective in absorbing volatile material from the FED cavity because of design considerations. The FED are usually large in area and the spacing between cathode and anode plate is usually quite small. For example, as shown in FIG. 1, the cavity spacing D between the cathode plate 50 and anode plate 10 is typically only about 200 micrometers while the length L, as depicted in FIG. 1, can be greater than 20 centimeters. The outgassing during operation of the FED predominantly occurs from the heating of the phosphors on the anode plate by the electrons emitted from the cathode. Therefore, the outgassing material in the cathode/anode region is not very effectively removed because of the narrow passageway and the remote location of the gettering material. One method of providing improved gettering efficiency is described by R. T. Longo, U.S. Pat. No. 5,063,323, in which additional interconnecting channels are formed between the base for the field emitters and the gate electrode, thus providing additional channels for the evolving gas to escape. However, the gettering material is formed on the peripheral inner walls of the Longo field emitter device and still a considerable distance from the outgassing surfaces. Therefore, local undesirable pressure increases can still occur during operation of the FED.
Another method of removing the outgassed materials form a FED is described by G. P. Kochanski, U.S. Pat. No. 5,283,500, in which the field emitters and gate electrodes are composed of gettering materials, such as tantalum, titanium, niobium or zirconium.
To understand the nature of the gettering method of G. P. Kochanski, a greatly enlarge schematic cross sectional view is shown in FIG. 2 of a portion of the prior art FED of FIG. 1. Shown in FIG. 2, is one of the many field emitters 20 formed on the cathode electrode 34 and one of the many phosphor pixels 13 on the anode plate 10. During operation of the FED, electrons are ejected from the emitter 20 by applying a bias (in volt) between the gate electrode 32 and cathode electrode 34. The electrons are ejected into the space 7 and accelerated by means of a more positive voltage on the conducting layer 36 on anode plate 10, and thereby strike the phosphor pixel 13, generating the luminous flux 44. The high energy electrons also heat the phosphor 13 and portions of the anode plate 10 and thereby dislodge the trapped volatile gas molecules from the anode material. The G. P. Kochanski method is to form the field emitter 20 and the gate electrode from gettering material.
However, because the field emitter work function should be as low as possible for good electron emission efficiency and should not change during the intended useful life of the FED, gettering by the field emitters can be undesirable. Therefore, there is still a strong need in the industry to improved the gettering in a FED without significantly increasing the manufacturing complexity.